1. Technical Field
The present disclosure relates to a thin film transistor substrate for an organic light-emitting diode display having two different type thin film transistors on the same substrate, a display using the same, and a manufacturing method thereof.
2. Discussion of the Related Art
Nowadays, as the information society develops, the requirements of displays for representing information are increasing. Accordingly, various flat panel displays (FPD) have been developed for overcoming many drawbacks of the cathode ray tube (CRT) display, which is heavy and bulky. The flat panel display devices include a liquid crystal display (LCD) device, a plasma display panel (PDP), an organic light-emitting diode (OLED) display device, and an electrophoresis display (ED) device.
The display panel of a flat panel display may include a thin film transistor substrate having a thin film transistor allocated in each pixel region arrayed in a matrix manner. For example, the liquid crystal display (LCD) device displays images by controlling the light transmissivity of the liquid crystal layer using the electric fields. The OLED displays images by forming an organic light-emitting diode at each pixel disposed in a matrix.
As a self-emitting display device, the organic light-emitting diode (OLED) display device has merit in very fast response speed, very high brightness, and a large viewing angle. The OLED display device using the organic light-emitting diode having the good energy efficiency can be categorized into an passive matrix type organic light-emitting diode (PMOLED) display and an active matrix type organic light-emitting diode (AMOLED) display.
FIG. 1 is a diagram illustrating a structure of a related art organic light-emitting diode. As shown in FIG. 1, the organic light-emitting diode includes an organic light-emitting material layer, and an cathode and an anode facing each other with the organic light-emitting material layer therebetween. The organic light-emitting material layer includes a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, and an electron injection layer EIL. The organic light-emitting diode radiates light due to energy from an exciton formed in an excitation state in which a hole (from the anode) and an electron (from the cathode) are recombined at the emission layer EML. The organic light-emitting diode display can represent video data by controlling the amount (or “brightness”) of the light generated and radiated from the emission layer EML of the organic light-emitting diode, as shown in FIG. 1.
The active matrix type organic light-emitting diode (AMOLED) display shows the video data by controlling the current applied to the organic light-emitting diode using the thin film transistor (TFT). FIG. 2 is an example of a related art circuit diagram illustrating a structure of one pixel in an active matrix organic light-emitting diode (AMOLED) display. FIG. 3 is a plane view illustrating a structure of one pixel in the AMOLED according to the related art. FIG. 4 is a cross-sectional view along the line I-I′ of FIG. 3 for illustrating the structure of the AMOLED according to the related art.
With reference to FIGS. 2 to 4, the active matrix organic light-emitting diode display includes a switching thin film transistor ST, a driving thin film transistor DT connected to the switching thin film transistor ST, and an organic light-emitting diode OLE connected to the driving thin film transistor DT. The switching thin film transistor ST is formed where the scan line SL and the data line DL cross. The switching thin film transistor ST selects the pixel that is connected to the switching thin film transistor ST. The switching thin film transistor ST includes a gate electrode SG branching from the gate line GL, a semiconductor channel layer SA overlapping the gate electrode SG, a source electrode SS, and a drain electrode SD.
The driving thin film transistor DT drives an anode electrode ANO of the organic light-emitting diode OLE disposed at the pixel selected by the switching thin film transistor ST. The driving thin film transistor DT includes a gate electrode DG connected to the drain electrode SD of the switching thin film transistor ST, a semiconductor channel layer DA, a source electrode DS connected to the driving current line VDD, and a drain electrode DD. The drain electrode DD of the driving thin film transistor DT is connected to the anode electrode ANO of the organic light-emitting diode OLE.
With further reference to FIG. 4, the gate electrode SG of the switching thin film transistor ST and the gate electrode DG of the driving thin film transistor DT are respectively formed on the substrate SUB of the related art active matrix organic light-emitting diode display. The gate insulator GI is deposited on the gate electrodes SG and DG. The semiconductor layers SA and DA are respectively formed on the gate insulator GI overlapping with the gate electrodes SG and DG. The source electrode SS and DS and the drain electrode SD and DD are respectively formed facing and separated from each other on the semiconductor layers SA and DA. The drain electrode SD of the switching thin film transistor ST is connected to the gate electrode DG of the driving thin film transistor DT via the contact hole penetrating the gate insulator GI. The passivation layer PAS is deposited on the substrate SUB having the switching thin film transistor ST and the driving thin film transistor DT.
In particular, when the semiconductor layers SA and DA include an oxide semiconductor material, due to the characteristics of high electron mobility, there is a possibility of providing a large area thin film transistor substrate having a large charging capacitor. However, to ensure the stability of the oxide semiconductor material, it is preferable to include an etch stopper SE and DE covering the upper surface of channel area to protect them from etchants. In detail, the etch stoppers SE and DE would be formed to protect the semiconductor layers SA and DA from being back-etched by the etchant for patterning the source electrodes SS and DS and the drain electrodes SD and DD.
A color filer is formed at the area where the anode electrode ANO will be formed later. It is preferable for the color filter CF to have as large an area as possible. For example, it is preferable to overlap the color filter CF with part of the data line DL, the driving current line VDD, and/or the scan line SL. The upper surface of the substrate having these thin film transistors ST and DT and color filters CF is not even and/or smooth, but is uneven and/or rugged, e.g., having many steps. Therefore, in the related art, to make the upper surface planar and with even conditions, an overcoat layer OC is deposited on the whole surface of the substrate SUB.
Then, the anode electrode ANO of the organic light-emitting diode OLE is formed on the overcoat layer OC. Here, the anode electrode ANO is connected to the drain electrode DD of the driving thin film transistor DT through the contact hole penetrating the overcoat layer OC and the passivation layer PAS.
On the substrate SUB having the anode electrode ANO, a bank BN is formed over the area having the switching thin film transistor ST, the driving thin film transistor DT, and the various lines DL, SL, and VDD, for defining the light-emitting area. The exposed portion of the anode electrode ANO by the bank BN would be the light-emitting area.
The organic light-emitting layer OL and the cathode electrode CAT are sequentially stacked on the anode electrode ANO exposed from the bank BN. When the organic light-emitting layer OLE has a material emitting white light, each pixel can represent various colors by the color filter CF disposed under the anode electrode ANO. The organic light-emitting diode display as shown in FIG. 4 is a bottom-emission type display in which visible light is radiated to the bottom direction of the display substrate.
For the organic light-emitting diode display, the organic light-emitting diode is operated using a large current. Therefore, it is preferable that the thin film transistor for driving the organic light-emitting diode would have large current-driving characteristics. For example, the oxide semiconductor material is adaptable. Recently, as the need for a high density organic light-emitting diode display is increasing, the thin film transistor is required to have the characteristics for driving large current with less power consumption.
The oxide semiconductor material has a disadvantage in that characteristics are easily degraded by the light induced from the environment, i.e., ambient light. For the organic light-emitting diode display, the light generated at the organic light-emitting layer may be introduced into the oxide semiconductor material. In that case, it is difficult to maintain the characteristics of the thin film transistor in best condition.
For the bottom-gate structure as shown in FIG. 4, the gate metal is disposed at the bottom side of the oxide semiconductor material where the light from the outside is intruding. Therefore, the gate metal can easily block the outside light. However, in the bottom-gate structure, the source electrode and the drain electrode directly contact the semiconductor layer. When forming the source and drain electrodes, the semiconductor layer has a back channel etched (BCE) structure in which some thickness of the channel area is etched. With the BCE structure, it is very difficult to ensure the reliability and/or stability of the semiconductor's characteristics.
Currently, many products for personal and portable information devices are being rapidly developed so that displays are being developed for portable or wearable devices. For portable or wearable devices, low power consumption displays are required. With currently-used technologies, it is difficult to manufacture the display having low power consumption specialized for the portable and/or wearable devices.